The tropical Pacific is not as flat as a pond. Instead, it regularly changes its boundaries from high to low. Natural cycles such as the El Niño and La Niña phenomena rock sea level back and forth, with an ocean near Asia at one end and an ocean adjacent to the Americas on the other. But over the past 30 years, swings have become more extreme, causing sea level fluctuations three times higher than in recent years. Why is this happening?
A new study from NASA University has found that the different formations of two separate climate cycles may be responsible for these intensifying fluctuations, which occur at the pinnacle of global sea level rise due to melting ice sheets and warming oceans. The findings will help improve forecasts of sea level changes, allowing vulnerable coastal communities to prepare for the risks of flooding, erosion and other hazards associated with sea level rise.
Tony Song of NASA's Jet Propulsion Laboratory, Pasadena, California, and colleagues looked at the correlations of the tropical Pacific sea level at different phases of two important climate cycles: the Pacific Decadal Oscillation (PDO) and El Niño / Southern Oscillation (ENSO). Song and his team found that the phases of these cycles can either amplify or weaken each other, directly affecting sea level variability across the Pacific.
Between 1990 and 2000, these sea level changes averaged about 6 inches (16 centimeters) - five times the global sea level rise over the same period. Asia is currently at the top of the sea level swing, while the coastline of the Americas, farther north of Southern California, benefits from lower sea levels. For communities threatened by rising sea levels, reverse swing movement is critical.
The two PDO phases and the two ENSO phases can be combined in four different ways. Just like flipping a dime and a dime together, you get four different combinations of heads and tails. Song and colleagues broke a 60-year record for each of the four combinations predominating in the tropical Pacific and compared to a record of observed east-west sea level fluctuations over the same period.
The correlation jumped between two of the four combinations and sea levels: El Niño plus positively correlated PDO wobble with high sea level in the Americas, and La Niña plus negatively correlated PDO wobble with high Asian sea level.
“These things are so well matched that we were very surprised,” said Jae Hoon Moon, lead author of a study article published in The Journal of Geophysical Research - Oceans. Moon did most of the research while working at the Jet Propulsion Laboratory (JPL), and is currently an Assistant Professor at Jeju National University, South Korea.
These newfound correlations provide a coherent answer to the question of why sea level fluctuations appear to have increased over the past decade. For the entire period from 1950 to 1980, the Pacific Ocean was in the negative PDO phase while the El Niño and La Niña phenomena occurred. This means that events can only happen in two out of four possible phase combinations. The study authors argue that when one of the two combinations of negative PDO wobble and El Niño occurs, their cycles counter each other, suppressing each other's effects on sea level that each would have individually.
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From 1980 to 2010, both positive and negative phases of PDO oscillations were observed in addition to the El Niño and La Niña phenomena. In fact, all four combinations of the two cycles could be observed at some point during this period. El Niño / PDO positive wobble and La Niña / negative PDO wobble occurred during this time period but were not seen in the previous 30 years. This increased sea level variability.
"Whether this increase in variability will continue is unclear," explained Dr. Song, because scientists still do not understand what exactly causes the phase change in any of the cycles. “We are delighted to have found another piece of the puzzle in the ongoing study of Pacific Ocean variability,” he concluded.
Sheremeteva Lesya